The present invention relates generally to illumination and more particularly to LED-based tubular substitutes for the neon lamp. The neon lamp is a venerable and ubiquitous accent light with such technological maturity that no significant improvements are in sight, but with such fragility and high operating voltage (>15,000 V) as to pose inescapable risks. For example, fragility is so high that long-distance shipping is not economical due to inevitable breakage, and thus most lamps are custom-produced locally by small businesses, precluding the economies of mass production. Of societal significance is the relatively low efficacy of neon lamps, leading to excessive power consumption. Furthermore, neon lamps are volume emitters, leading to their perceived brightness being proportional to the length of the viewer's line of sight through the cylinder. Thus they are highly non-uniform spatially and directionally.
Fluorescent tubes have uniform brightness but lack neon's color purity.
LEDs have better efficacy and can generate white light of selectable color temperature, light of higher color purity than fluorescent tubes, or programmably multicolored light, with great improvements in safety, robustness, and operating life as compared with neon technology. Recent gains in the luminosity and operating lifetime of light-emitting diodes (LEDs) make them attractive candidates to replace neon light sources, especially considering their robustness and low operating voltage. The prior art includes numerous ways of end-firing LEDs into long runs of plastic illumination fibers, typically 0.5-1 inch (12 to 25 mm) diameter. This sets rather low limits on the output brightness of the fiber.
Relative to the size of neon lamps, however, LEDs are point sources, so that their light must be somehow spread out along a tubular structure so as to reproduce the look of neon. The end-fired cylindrical lights of the prior art, such as U.S. Pat. No. 6,488,397 by Matsutani & Ishiharata are limited in the amount of light that can be injected into them, which limits their length as well, if they are to equal or surpass the luminance of neon lamps. Also, many illumination fibers are made of elastomeric material, so that their lack of rigidity requires frequent support to prevent unsightly sagging. Finally, the diameter of such solid fibers is limited by weight considerations.
In the recent prior art, U.S. Pat. No. 7,048,413 by Fan discloses a round scattering layer above a line of bullet-lens LEDs. This approach however, is not a true tube-emitter like neon, since its light shines only out the front part of the device. Also, the bullet-lens LEDs it utilizes represent an older configuration that is optically inefficient and generally of lower efficacy than the more recent high-brightness LED configurations utilized in the present application. U.S. Pat. No. 6,834,979 by Cleaver et al is similar in its approach and limitations.
U.S. Pat. No. 6,874,924 by Hulse & Chambers discloses an annular cylindrical structure cut so as to allow side injection of light, again by bullet-lens LEDs, but also including an interior partially reflecting scattering layer. U.S. Pat. No. 6,497,496 by Wang also utilizes a non-waveguiding annular cylinder with radially directed light sources and opposing holes. U.S. Pat. No. 6,676,284 by Wilson also has an annular cylinder, but it is merely a diffusing cover illuminated by a line of centrally located, closely spaced light sources.
U.S. Pat. No. 7,277,618 by Yamazaki, et al discloses a conventional optical fiber impregnated with phosphor and end-fired by blue LEDs. Impregnating the waveguide itself with phosphor would make it difficult to attain uniformity of either brightness or whiteness, because blue light is being converted to yellow as it propagates down the fiber, so that its strength weakens and the local fiber emission diminishes and alters color.
There is a great need for a superior LED-based neon substitute that can utilize the latest high-brightness LEDs (which are not available in the thermally inefficient bullet lenses mention above).